Tour:Union of two subgroups is not a subgroup unless they are comparable
This article adapts material from the main article: union of two subgroups is not a subgroup unless they are comparable
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We just saw that an intersection of subgroups is always a subgroup. The corresponding statement isn't true for unions.
WHAT YOU NEED TO DO:
- Read, and try to prove, the statements below
- Read the proof after you've tried
- Try to come up with an example (think of the group of integers, for instance) to illustrate this.
Statement
Suppose is a group and
and
are subgroups of
. Then, the following are true:
- If
is a subgroup of
contained in
, then
is contained either in
or in
(note: it may be contained in both).
-
is a subgroup of
if and only if either
is contained in
or
is contained in
. (Note that if
is contained in
,
, and if
is contained in
,
).
Proof
Proof idea
The proof is based on the idea that if two of the elements belong to a subgroup, so does the third. We carry out the proof by contradiction starting with
and
in different pieces, and show that whichever piece the product
lands in, we get a contradiction.
The key use of us working with groups arises in the step where we need to deduce that lying in the same subgroup as one of the elements
or
implies the other element (respectively,
or
) also lies in the same subgroup.
Proof details for (1)
This proof uses a tabular format for presentation. Provide feedback on tabular proof formats in a survey (opens in new window/tab) | Learn more about tabular proof formats|View all pages on facts with proofs in tabular format
Given: A group , subgroups
such that
To prove: is contained in
or
is contained in
.
Proof: We use the technique of proof by contradiction. Below is the assumption from which we will try to derive a contradiction.
ASSUMPTION: is not contained in
and
is not contained in
.
Step no. | Assertion/construction | Facts used | Given data/assumptions used | Previous steps used | Explanation |
---|---|---|---|---|---|
1 | We can find ![]() ![]() ![]() ![]() ![]() ![]() |
![]() ![]() ![]() ![]() |
|||
2 | ![]() ![]() |
![]() ![]() |
Step (1) | We know that ![]() ![]() ![]() ![]() ![]() ![]() | |
3 | ![]() |
![]() ![]() |
Step (1) | By Step (1), ![]() ![]() ![]() ![]() | |
4 | ![]() ![]() |
![]() ![]() |
Step (3) | ![]() ![]() ![]() ![]() ![]() | |
5 | If ![]() ![]() ![]() ![]() |
![]() ![]() ![]() |
Step (2) | ![]() ![]() ![]() ![]() Similarly, ![]() ![]() ![]() ![]() | |
6 | We have the desired contradiction. | Steps (1), (4), (5) | By Step (4) and (5), either ![]() ![]() ![]() ![]() |
Proof of (2): direction from union being a subgroup to comparability
Given: Group , subgroups
of
such that
is a subgroup of
.
To prove: Either or
.
Proof: We set in part (1) of the statement to get that
or
. The former possibility is equivalent to
and the latter possibility is equivalent to
. Thus, one of the two must hold.
Proof of (2): direction from comparability to union being a subgroup
Given: Group , subgroups
of
such that
or
To prove: is a subgroup of
Proof: If , then
, which we know is a subgroup. If
, then
, which we know is a subgroup. Thus, in either case,
is a subgroup.
PONDER (WILL BE EXPLORED LATER IN THE TOUR):WHAT'S MORE: Below are listed some related facts, counterexamples for more than two subgroups, and the relation with a result in commutative algebra. Ignore the parts that use unfamiliar terminology.
- What are the key facts about the structure of groups that we're using here?
- For what other weaker structures (magma, monoid, semigroup, quasigroup) does the argument go through?
Statement (slightly more general formulation)
Suppose is a group and
and
are subgroups of
. Then, the following are true:
- If
is a subsemigroup of
contained in
, then
is contained either in
or in
(note: it may be contained in both).
-
is a subsemigroup of
if and only if either
is contained in
or
is contained in
. (Note that if
is contained in
,
, and if
is contained in
,
).
Related facts
Conditions for the existence of two incomparable subgroups
This result is of interest only when we have examples of a group and subgroups
and
of
such that neither is contained in the other. Here are some examples:
- In the group of integers
under addition, the subgroups generated by any two integers, neither of which is a divisor of the other, are not comparable. This is because subgroup containment relation in the group of integers equals divisibility relation on generators. In particular, the subgroups of multiples of
and multiples of
have the property that neither is contained in the other.
- In the direct product of two nontrivial groups, neither direct factor is contained in the other.
On the other hand, in a cyclic group of prime power order, any two subgroups are comparable. In fact, the only finite groups where any two subgroups are comparable are the trivial group and the cyclic groups of prime power order; the only infinite groups with the property are the quasicyclic groups.
A group where any two normal subgroups are comparable in termed a normal-comparable group: there are more examples of normal-comparable groups.
Tightness
It is possible for a union of three subgroups to be a subgroup; for instance, the Klein four-group (which is a direct product of the cyclic group of order two with itself) is a union of three of its subgroups. This is more generally related to the notion of partition of a group: a way of expressing a group as a union of proper subgroups any two of which intersect in the trivial subgroup.
In general, the elementary abelian group of order (defined as the direct product of two cyclic groups of order
) is a union of
subgroups of order
, any two of which intersect trivially.
We do have a fairly tight restriction on how a group may be a union of three subgroups and somewhat weaker restrictions on other unions of finitely many subgroups:
- Union of three subgroups is the whole group implies they have index two and form a flower arrangement
- Union of n subgroups is the whole group iff the group admits one of finitely many groups as quotient
- There is no group that is a union of seven proper subgroups but not a union of fewer proper subgroups
- B. H. Neumann's lemma
Related facts in group theory
- Directed union of subgroups is subgroup
- Union of all conjugates is proper
- Every group is a union of cyclic subgroups
- Every group is a union of maximal among abelian subgroups
- Cyclic iff not a union of proper subgroups
Analogues in other algebraic structures
- A closely related result in commutative algebra is the prime avoidance lemma. The prime avoidance lemma states that a prime ideal is contained in a union of finitely many ideals, at most two of which are not prime, if and only if it is contained in one of them. In fact, the prime avoidance lemma for two ideals is precisely this statement, and the case of two ideals is the starting point for the argument.
- [[Union of two subquasigroups is not a subquasigroup unless they are comparable]]: The analogous result holds for subquasigroups of a quasigroup. The key reason why the result holds is that it continues to be true that if two of the elements
are in the subquasigroup, so is the third.
- Union of two incomparable submonoids may be a submonoid: For monoids, we can have two submonoids, neither of which is contained in the other, but their union is a submonoid. Note that it is still true that in most cases, the union is not a submonoid; however, the possibility cannot be ruled out. For instance, the union of the non-negative integers and the non-positive integers is the group of integers. Both the non-negative integers and the non-positive integers form submonoids, and neither is contained in the other.
This page is part of the Groupprops Guided tour for beginners (Jump to beginning of tour). If you found anything difficult or unclear, make a note of it; it is likely to be resolved by the end of the tour.
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